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The end of Acromyrmex?

Those of you familiar with South American ants know the distinction between the two great lineages of leafcutters. Acromyrmex comprises the smaller, stubbier, spinier species, while Atta contains the larger, leggier, and less spiny species. This dichotomy is marked enough even to be recognized in the local vernacular. The Guaraní called the former Akêkê and the latter Ysaú, which Brazilians have modified into the Portuguese Quemquem and Sauva. When preliminary phylogenetic work appeared to confirm the duality, I was pleased by the concordance of cultural and phylogenetic classifications.

Modified from Figure 4. Bayesian inference phylogenetic tree based on the concatenated sequences of four nuclear genes (WG, LW, EF1?F1 and EF1?F2).

A new study in PLoS One by Maykon Passos Cristiano et al added the unusual species Acromyrmex striatus to the phylogenetic framework, and- surprise!- it’s not like the others. Acromyrmex striatus chromosomes appear more similar to those of Atta than to those of its own congeners, and a phylogenetic reconstruction based on several protein-coding genes suggests this species may be a third, distinct lineage.

The authors state that more sampling is needed. Fair enough. But based on these data I would not be surprised if this taxonomic mismatch is eventually resolved by condensing leafcutters into a single monophyletic Atta.

9 comments on The end of Acromyrmex?

BenGH

Why not just split A. striatus into a new genus, Pseudacromyrmex? Apologies if the authors proposed this. No time to look at the paper yet. Seems a shame to lump genera if they are so clearly diagnosable.

Awesome, but oh man! I want to see more Trachymyrmex species in this tree! The males do unbelievable things (in terms of genitalia), with simple “boring” or highly derived “gee-whiz” genitalia. I have seen no intergradation thus far between the complex and simple forms. Trachymyrmex is further notable because of the wild variation of seemingly informative gross morphology up to, and including presence or absence of the pterostigma. Anyway .

Agreed! We know from earlier molecular phylogenetic work (Shultz & Brady) that Trachymyrmex is itself paraphyletic, with T. septentrionalis and relatives coming out closer to the leafcutters than to other Trachys. If the arrangement shown here is a long-branch artifact, then including those Trachys may help change the topology, too.

This is in stark contrast to a paper in press by Schultz and Brady which has Atta branching off basal to all of the Acromyrmex. So don’t get too excited yet. This hasn’t been resolved. Besides, I always say everything changes every 5 years and everything you know is wrong. 🙂

All I know is from a cladogram on a poster Ted hung on the wall, probably from a presentation at a recent meeting. (I work on the same floor as Ted and Sean.) I didn’t see A. striatus/A. silvestri among the species they analyzed. That seems to be where the difference is! I also noticed that a new species of Pseudoatta came out among the Acromyrmex, so what does that mean for the monophyly of Acromyrmex?

Looking at the PlosOne paper, I don’t see how this really challenges any type of monophyly or new view of the Atta and Acromyrmex designations. If you look at figure 6, not only does A.striatus have the same number of chromosomes as Atta, it also has the same type of tuberculate morphology on the abdomen as Atta.

A.striatus is just the outgroup of Atta and Acromyrmex i.e.: it is neither Atta nor Acromyrmex. Atta simply reduced its spine morphology while the Acromyrmex group evolved the abdominal tuberculate and increased chromosome number. So, I don’t see how this renders the Acronomyrmex designation extinct. There is this idea of a problem because taxonomists seem to be clinging to the idea that A.striatus is in fact an Acromyrmex which would mean paraphyly of this grouping…but it just isn’t Acromyrmex, rather, it represents the common ancestor of the two groups